The present invention is directed to servo control systems, and in particular, to velocity servo systems. The specific application of the present invention is a servo system control for positioning a vertical-travel paper picker for a multi-input-bin printing apparatus.
Servo systems to control positioning an object such as the paper picker for a printer apparatus are of two general types--position servos and velocity servos.
Position servos utilize position and velocity feedback to obtain high resolution (i.e. precise positional and translational control), including the ability to continuously hold a particular target position (zero velocity). However, such position servos exhibit a number of disadvantages. Analog implementations require extensive analog hardware to develop reliable analog position information, including cumbersome calibration circuitry. In digital implementations, executing the algorithm required to calculate velocity and acceleration for precise positional control requires extensive processing time.
Digital velocity servos require significantly less processor time than digital position servos. The processor merely provides a servo current-control set point based upon relative distance from target position (typically by a look-up-table operation) to control a high gain amplifier in conjunction with velocity feedback. However, velocity servos are disadvantageous in that holding a target position (zero velocity) is problematic. When a particular target position is achieved, positional deviations unavoidably occur (such as from electronic drift) resulting in a positional error. While this positional error is relatively small, the resulting velocity derivative in the velocity feedback loop is relatively large (i.e. the velocity servo exhibits low resolution at the target position. Because of the relatively high gain of the amplifier (required for accurate translational movement), the set point correction for such positional deviations result in unavoidable target overshoot that causes oscillation about the target position.
For example, to reposition a vertical-travel paper picker the current-control set point is gradually decreased until, at the target position, the set point goes to zero. However, drift in the control electronics and gravity cause deviation from target position. The resulting compensating velocity derivative of this positional deviation in the velocity feedback loop causes continual overshoot such that the servo motor is alternately driven positively and negatively, putting excessive strain on the power drive and needlessly dissipating power in the motor to maintain position.